Stamped fuel cell bipolar plate

ABSTRACT

A bipolar plate assembly for a fuel cell having a pair of stamped plates joined together to define a coolant volume therein. Each of the pair of stamped plates have a flow field arranged to maximize the contact area between the plates while allowing coolant to distribute and flow readily within the coolant volume. The bipolar plate assembly further includes a seal arrangement and integral manifold to direct gaseous reactant flow through the fuel cell.

TECHNICAL FIELD

This invention relates to a fuel cell stack assembly and moreparticularly to a bipolar plate assembly having a pair of stamped metalplates bonded together to provide coolant volume therebetween.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for many applications.One such fuel cell is the proton exchange membrane or PEM fuel cell. PEMfuel cells are well known in the art and include an each cell thereof aso-called membrane-electrode-assembly or MEA having a thin, protonconductive, polymeric membrane-electrolyte with an anode electrode filmformed on major face thereof and a cathode electrode film formed on theopposite major face thereof. Various membrane electrolytes are wellknown in the art and are described in such as U.S. Pat. Nos. 5,272,017and 3,134,697, as well as in the Journal of Power Sources, vol. 29(1990) pgs. 367-387, inter alia.

The MEA is interdisposed between sheets of porous gas-permeable,conductive material known as a diffusion layer which press against theanode and cathode faces of the MEA and serve as the primary currentcollectors for the anode and cathode as well as provide mechanicalsupport for the MEA. This assembly of diffusion layers and MEA arepressed between a pair of electronically conductive plates which serveas secondary current collectors for collecting the current from theprimary current collectors and for conducting current between adjacentcells internally of the stack (in the case of bipolar plates) andexternally of the stack (in the case of monopolar plates at the end ofthe stack). Secondary current collector plates each contain at least oneactive region that distributes the gaseous reactants over the majorfaces of the anode and cathode. These active regions also known as flowfields typically include a plurality of lands which engage the primarycurrent collector and define therebetween a plurality of grooves or flowchannels through which the gaseous reactant flow between a supply headerand a header region of the plate at one of the channel and an exhaustheader in a header region of the plate at the other end of the channel.In the case of bipolar plates, an anode flow field is formed on a firstmajor face of the bipolar plate and a cathode flow field is formed on asecond major face opposite the first major face. In this manner, theanode gaseous reactant (e.g., H₂) is distributed over the surface of theanode electric film and the cathode gaseous reactant (e.g., O₂/air) isdistributed over the surface of the cathode electrode film.

The various concepts of been employed to fabricate a bipolar platehaving flow fields formed on opposite major faces. For example, U.S.Pat. No. 6,099,984 discloses bipolar plate assembly having a pair ofthin metal plates with an identical flow field stamped therein. Thesestamped metal plates are positioned in opposed facing relationships witha conductive spacer interposed therebetween. This assembly of plates andspacers are joined together using conventional bonding technology suchas brazing, welding, diffusion bonding or adhesive bonding. Such bipolarplate technology has proved satisfactory in its gas distributionfunction, but results in a relatively thick and heavy bipolar plateassembly and thus impacts the gravimetric and volumetric efficiency ofthe fuel cell stack assembly.

In another example, U.S. Pat. No. 6,503,653 discloses a single stampedbipolar plate in which the flow fields are formed in opposite majorfaces thereof to provide a non-cooled bipolar plate. A cooled bipolarplate using this technology again requires a spacer element interposedbetween a pair of stamped plates, thereby increasing the thickness andweight of the cooled plate assembly. U.S. Pat. No. 6,503,653 takesadvantage of unique reactant gas porting and staggered seal arrangementsfor feeding the reactant gases from the header region through the portin the plate to the flow field formed on the opposite side thereof. Thisconcept is very desirable in terms of cost but its design constraints onflow fields may rule out some application. Furthermore, this designconcept does not lend itself readily to providing an internal coolingflow.

Applications with high powered density requirements need cooling inabout every other fuel cell. Thus, there is an ever present desire torefine the design of a bipolar plate assembly to be efficiently used ina fuel cell stack to provide a high gravimetric power density, highvolumetric power density, low cost and higher reliability. The presentinvention is directed to a stamped fuel cell bipolar plate that offerssignificant flow field design flexibility while minimizing the weightand thickness thereof.

SUMMARY OF THE INVENTION

The present invention is directed to a bipolar plate assembly having twothin metal plates formed with conventional stamping processes and thenjoined together. In another aspect, the centerlines of the flow fieldsmust be arranged to align the channels for plates on opposite sides ofthe MEA wherever possible to further provide uniform compression of thediffusion media. In another aspect, the configuration of the flow fieldsformed in each of the two stamped metal plates are such that the contactarea therebetween is maximized to enable the bipolar plate assembly tocarry compressive loads present in a fuel cell stack. Thus, thecenterlines of the flow fields formed in the two thin metal plates of abipolar plate assembly need to be coincident in many places to carry thecompressive loads. However, since the interior volume defined betweenthe plates and their context areas form an interior cavity for coolantflow, it is necessary to have sufficient instances where the centerlinesare not coincident in order to allow adequate coolant flow. The presentinvention achieves these two apparently opposing objections with aunique flow field design in which ajoining areas of the flow channelsadjacent the inlet and exhaust margins provide a geometricconfigurations to provide the desired flow field and contact arearequirements.

The present invention provides a bipolar plate assembly which includes apair of plates having reactant gas flow fields defined by a plurality ofchannels formed the outer faces of the plates. The plates are arrangedin a facing relationship to define an interior volume therebetween. Acoolant flow field is formed in an interior volume defined between thepair of plates at the contact interface therebetween. The coolant flowfield has an array of discrete flow disruptors adjacent a coolant headerinlet and a plurality of parallel channels interposed between the arrayand the coolant exhaust header. Fluid communication is provided from thecoolant inlet header through the coolant flow field to the coolantexhaust header.

The present invention also provides a separator plate which includes athin plate having an inlet margin with a pair of lateral inlet headersand a medial inlet header formed therethrough, an exhaust marginincluding a pair of lateral exhaust headers and a medial exhaust headerformed therethrough and a reactant gas flow field formed on a major faceof the thin plate. The reactant gas flow field includes a first set offlow channels, each having an inlet leg with a first longitudinalportion in fluid communication with one of the pair of lateral inletheaders and a first transverse portion, a serpentine leg having a firstend in fluid communication with the first transverse portion and asecond end and an exhaust leg having a second transverse portion influid communication with the second end of the serpentine leg and asecond longitudinal portion in fluid communication with one of the pairof lateral exhaust headers. Either of the transverse portion of theinlet leg adjacent the medial inlet header and the transverse portion ofthe exhaust leg adjacent the medial exhaust header may be defined by anundulating flow channel.

These and other aspects of the present invention provide a bipolar plateassembly which increases the design flexibility in terms of flow fieldoptions, while achieving the cooling requirements as well as providing arelatively high gravimetric power density and high volumetric powerdensity from a fuel cell stack incorporating the bipolar plate assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood when considered in the light ofthe following detailed description of a specific embodiment thereofwhich is given hereafter in conjunction with the several figures inwhich:

FIG. 1 is a schematic isometric exploded illustration of a fuel cellstack;

FIG. 2 is an isometric exploded illustration of a bipolar plate assemblyand seal arrangement in accordance with the present invention;

FIG. 3 is a plan view of the flow field formed in the major face of ananode plate in the bipolar plate assembly shown in FIG. 2;

FIG. 4 is a plan view of the flow field formed in the major face of acathode plate in the bipolar plate assembly shown in FIG. 2;

FIG. 5 is a plan view showing the contact areas at the interface betweenthe anode and cathode plates;

FIG. 6 is an isometric view of multiple cells within the fuel cell stackand further showing a section taken through the cathode header;

FIG. 7 is a cross-section taken through the coolant header and showingthe coolant flow path;

FIG. 8 is a cross-section taken through the anode header and showing theanode gas flow path; and

FIG. 9 is a cross-section taken through the cathode header and showingthe cathode flow path.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the preferred embodiment is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. With reference to FIG. 1, a two-cell stack(i.e., one bipolar plate) is illustrated and described hereafter, itbeing understood that a typical stack will have many more such cells andbipolar plates. FIG. 1 depicts a two-cell bipolar PEM fuel cell stack 2having a pair of membrane-electrode-assemblies (MEAs) 4, 6 separatedfrom each other by an electrically conductive, liquid-cooled bipolarplate 8. The MEAs 4, 6 and bipolar plate 8 are stacked together betweenclamping plates 10, 12 and monopolar end plates 14, 16. The clampingplates 10, 12 are electrically insulated from the ends plate 14, 16. Theworking face of each monopolar end plates 14, 16, as well as bothworking faces of the bipolar plate 8 contain a plurality of grooves orchannels 18, 20, 22, 24 defining a so-called “flow field” fordistributing fuel and oxidant gases (i.e., H₂ and O₂) over the faces ofthe MEAs 4, 6. Nonconductive gaskets 26, 28, 30 and 32 provide seals andelectrical insulation between the several components of the fuel cellstack. Gas-permeable diffusion media 34, 36, 38, 40 press up against theelectrode faces of the MEAs 4, 6. The end plates 14, 16 press up againstthe diffusion media 34, 40 respectfully, while the bipolar plate 8presses up against the diffusion media 36 on the anode face of MEA 4,and against the diffusion media 38 on the cathode face of MEA 6.

With reference to FIG. 2, the bipolar plate assembly 8 includes twoseparate metal plates 100, 200 which are bonded together so as to definea coolant volume therebetween. The metal plates 100, 200 are made asthin as possible (e.g., about 0.002-0.02 inches thick) and arepreferably formed by suitable forming techniques as is known in the art.Bonding may, for example, be accomplished by brazing, welding diffusionbonding or gluing with a conductive adhesive as is well known in theart. The anode plate 100 and cathode plate 200 of a bipolar plateassembly 8 are shown having a central active region that confronts theMEAs 36, 38 (shown in FIG. 1) and bounded by inactive regions ormargins.

The anode plate 100 has a working face with an anode flow field 102including a plurality of serpentine flow channels for distributinghydrogen over the anode face of the MEA that it confronts. Likewise, thecathode plate 200 has a working face with a cathode flow field 202including a plurality of serpentine flow channels for distributingoxygen (often in the form of air) over the cathode face of the MEA thatit confronts. The active region of the bipolar plate 8 is flanked by twoinactive border portions or margins 104, 106, 204, 206 which haveopenings 46-56 formed therethrough. When the anode and cathode plates100, 200 are stacked together, the openings 46-56 in the plates 100, 200are aligned with like openings in adjacent bipolar plate assemblies.Other components of the fuel cell stack 2 such as gaskets 26-32 as wellas the membrane of the MEAs 4 and 6 and the end plates 14, 16 havecorresponding openings that align with the openings in the bipolar plateassembly in the stack, and together form headers for supplying andremoving gaseous reactants and liquid coolant to/from the stack.

In the embodiment shown in the figures, opening 46 in a series ofstacked plates forms an air inlet header, opening 48 in series ofstacked plates forms an air outlet header, opening 50 in a series ofstacked plates forms a hydrogen inlet header, openings 52 in a series ofstacked plates forms a hydrogen outlet header, opening 54 in a series ofstacked plates forms a coolant inlet header, and opening 56 in a seriesof stacked plates forms a coolant outlet header. As shown in FIG. 1,inlet plumbing 58, 60 for both the oxygen/air and hydrogen are in fluidcommunication with the inlet headers 46, 50 respectively. Likewise,exhaust plumbing 62, 64 for both the hydrogen and the oxygen/air are influid communication with the exhaust headers 48, 52 respectively.Additional plumbing 66, 68 is provided for respectively supplying liquidcoolant to and removing coolant from the coolant header 54, 56.

FIG. 2 illustrates a bipolar plate assembly 8 and seals 28, 30 as theyare stacked together in a fuel cell. It should be understood that a setof diffusion media, an MEA, and another bipolar plate (not shown) wouldunderlie the cathode plate 200 and seal 30 to form one complete cell.Similarly, another set of diffusion media and MEAs (not shown) willoverlie the anode plate 100 and seal 28 to form a series of repeatingunits or cells within the fuel cell stack. It should also be understoodthat an interior volume or coolant cavity 300 is formed directly betweenanode plate 100 and cathode plate 200 without the need of an additionalspacer interposed therebetween.

Turning now to FIG. 3, a plan view of the anode plate 100 is providedwhich more clearly shows the anode flow field 102 formed in the workingface of anode plate 100. As can also be clearly seen in FIG. 3, theinlet margin 104 of anode plate 100 has a pair of lateral inlet headers46 and 50 to transport cathode gas and anode gas, respectively, throughthe fuel cell stack and a medial inlet header 54 to transport a coolantthrough the stack. Similarly, the exhaust margin 106 has a pair oflateral exhaust headers 48, 52 for transporting anode affluent andcathode affluent, respectively through the fuel cell stack, and a medialexhaust header 56 for transporting coolant through the fuel cell stack.

The anode flow field 102 is defined by a plurality of channels formed toprovide fluid communication along a tortuous path from the anode inletheader 50 to the anode exhaust header 52. In general, the flow channelsare characterized by an inlet leg 108 having a longitudinal portion 110with a first end in fluid communication with the anode inlet header 50and a second end in fluid communication with a transverse portion 112.As presently preferred, the transverse portion 112 of the inlet leg 108branches to provide a pair of transverse inlet legs associated with eachlongitudinal portion 110. Furthermore, the path of these transverseinlet portions 112 undulate within the plane of the anode plate 100 toprovide an undulating flow channel adjacent the coolant inlet header 54as represented in the area designated 114. The transverse portion 112 ofinlet leg 108 is in fluid communication with a serpentine leg 116. Theflow channel 108 further includes an exhaust leg 118 having transverseportions 120 and a longitudinal portion 122 to provide fluidcommunication from the serpentine leg 116 to the anode exhaust header52. The exhaust leg portion 118 is configured similar to the inlet legportion 108 in that each longitudinal portion 122 is associated with apair of transverse portions 120. The path of the transverse exhaustportions 120 undulate within the plane of the anode plate 100 to providean undulating flow channel adjacent the coolant exhaust header 56 asrepresented in the area designated 124.

Turning now to FIG. 4 a plan view of the cathode plate 200 is providedwhich more clearly shows the cathode flow field 202 formed in theworking face of cathode plate 200. As can also be clearly seen in FIG.4, the inlet margin 204 of cathode plate 200 has a pair of lateral inletheaders 46, 50 to transport cathode gas and anode gas, respectively,through the fuel cell stack and a medial inlet header 54 to transport acoolant through the stack. Similarly, the exhaust margin 206 has a pairof lateral exhaust headers 48, 52 for transporting anode affluent andcathode affluent, respectively through the fuel cell stack, and a medialexhaust header 56 for transporting coolant through the fuel cell stack.

The cathode flow field 202 is defined by a plurality of channels formedto provide fluid communication along a tortuous path from the cathodeinlet header 46 to the cathode exhaust header 48. In general, the flowchannels are characterized by an inlet leg 208 having a longitudinalportion 210 with a first end in fluid communication with the cathodeinlet header 46 and a second end in fluid communication with atransverse portion 212. A single transverse portion 212 is associatedwith each longitudinal portion 210. Thus, the transverse portion 212 ofthe inlet leg 208 does not branch off to provide a pair of transverseinlet portions as the transverse portion 112 of anode inlet leg 108. Thepath of the transverse inlet portions 212 undulate within the plane ofthe cathode plate to provide an undulating flow channel adjacent thecoolant inlet header 54 as represented in the area designated 214. Theflow channel further includes a serpentine leg 216 which is in fluidcommunication with the end of transverse inlet portion 212. The flowchannel further includes an exhaust leg 218 having a transverse portion220 and a longitudinal portion 222. The exhaust leg portion 218 isconfigured similar to the inlet leg portion 208 to provide fluidcommunication from the serpentine leg 216 to the cathode exhaust header48. The path of the transverse exhaust portions 220 undulate within theplane of the cathode plate to provide an undulating flow channeladjacent the coolant exhaust header 56 as represented in the areadesignated 224.

Referring now to FIGS. 2 and 6, the anode plate 100 and the cathodeplate 200 are positioned in an opposed facing relationship such that thevarious inlet and exhaust headers are in alignment. The anode plate 100and the cathode plate 200 are then joined together using conventionaltechniques. The centerlines of the anode flow fields 102 and cathodeflow fields 202 are arranged to align the flow channels on opposingplates (e.g. on opposite sides of the MEA as shown in FIG. 6) whereverpossible to provide uniform compression of the diffusion media and theMEA. Likewise, the contact area between the adjacent, joined anode plate100 and cathode plate 200 (as shown in FIG. 2) are coincident in manyplaces so as to carry the compressive loads imposed on the fuel cellstack. Specifically, the flow channels of anode flow field 102 formed inthe working face of anode plate 100 provide a complimentary contactsurface on an inner face opposite the working face. Similarly, the flowchannels of the cathode flow field 202 formed in the working face of thecathode plate 200 define a contact surface on an inner face of thecathode plate 200. Thus, when the anode plate 100 and cathode plate 200are joined together, an interference or contact area is definedtherebetween.

With reference now to FIG. 5, the contact area between the anode plate100 and the cathode plate 200 defines a coolant flow field 302 betweenan inlet margin 304 and an exhaust margin 306 within coolant cavity 300.The coolant flow field 302 includes an array of discrete flow disruptors308 adjacent the coolant inlet manifold 54 formed at the interface ofthe anode inlet legs 108 and the cathode inlet legs 208. Similarly, aset of flow disrupters 310 are formed adjacent the coolant exhaustheader 56 at the interface of the anode exhaust leg 118 and the cathodeexhaust legs 218. The coolant flow field 302 further includes aplurality of parallel of flow channels 312 interposed between the inletmargin 304 and the exhaust margin 306 which are defined at the interfaceof the serpentine legs 116 and the serpentine legs 216. In accordancewith the configuration of the anode flow field 102 and cathode flowfield 202, the array of discrete flow disruptors 308 extend obliquelyfrom the area of the coolant flow field 302 adjacent the coolant inletheader 54 as indicated by directional arrow 314 into the parallel flowchannels 312. Likewise, the array of discrete flow disruptors 310 extendfrom the parallel flow channels 312 obliquely towards the coolantexhaust header 56 as indicated by directional arrow 316.

Turning now to FIGS. 6-9, the present invention incorporates a staggeredseal and an integral manifold configuration for directing fluidcommunication from the header into the appropriate flow field. Forexample, the location of the seal beads between the inlet margin 104,204 and the flow field structure 102, 202 step left and right (as seenin FIGS. 7-9) for each successive layer. Thus, the seal position shiftsto provide fluid communication therebetween. Ports in the form of holesor slots penetrate vertically through the anode plate 100 or cathodeplate 200 to provide means for fluid communication from the header tothe flow field. In this manner, the present invention employs astaggered seal concept similar to that disclosed in U.S. Pat. No.6,503,653, which is commonly owned by the assignee of the presentinvention and whose disclosure is expressly incorporated by referenceherein. This approach allows the combined seal thicknesses to equal therepeat distance minus the thickness of the anode plate and cathodeplate. This approach also provides an advantage over other conventionalfuel cell stack design in which the thickness available for seals isreduced by the height required for the fluid passage from the headerregion to the active area region. By utilizing a staggered seal concept,the present invention affords the use of thicker seals which are lesssensitive to tolerance variations.

The present invention further improves upon the staggered seal conceptdisclosed in U.S. Pat. No. 6,503,653 with the use of separate anodeplate 100 and cathode plate 200 in each bipolar plate assembly.Specifically, a second plate enables the use of an integral manifoldwith the space between the plates. Reactant gases can now enter on thetop side of the upper plate, travel between the upper and lower platethrough such integral manifolds and then enter the lower side of theupper plate to feed the bottom side of the MEA. Similarly, coolant fluidcan now enter on the top side of the upper plate, travel between theupper and lower plate through such integral manifolds and then enter thecoolant flow field. As a result, the width of the region where thereactant gases and coolant fluid enter the flow field is twice as wideas that disclosed in U.S. Pat. No. 6,503,653, thereby lowering theoverall pressure drop across a given flow field. This aspect of thepresent invention is best illustrated in FIGS. 7-9. Specifically, asillustrated in FIG. 7, the coolant flow path is indicated by the arrowsA showing flow from the coolant header (not shown) between the anodeplate 100 and the cathode plate 200 and into the coolant flow field 302defined therebetween. Similarly, in FIG. 8 the anode gas flow path isindicated by the arrows B showing flow from the anode header (not shown)between the cathode plate 200 and the anode plate 100 and into the anodeflow field 102. Similarly, in FIG. 9 the cathode gas flow path isindicated by the arrows C showing flow from the cathode header (notshown) between the anode plate 100 and the cathode plate 200 and intothe cathode flow field 202. In this manner, a wider manifold region isprovided between the header region and the flow field region for each ofthe fluids passed through the fuel cell stack.

As presently preferred, the design of the bipolar plate assembly furtherincludes an additional feature to support the seal loads given theeffect of widening the inlet manifold region between the headers and theactive flow fields. Specifically, as best seen in FIGS. 4 and 6 anin-situ support flange 226 extends transversely across the inlet marginthrough the cathode inlet header 46, the coolant inlet header 54 and theanode header 50. This support flange 226 is formed with a wavy orcorrugated configuration to allow inlet fluids to freely pass from theheader region through the manifold region into the flow field regionwhile at the same time providing through plane support for the bipolarplate assembly. For example, as best seen in FIG. 6, the support flange226 for the cathode plate 200 of the bipolar plate 8 occurs directlyover the support flange 126 for the anode plate 100 of the neighboringcell. In this manner, compressive loads are readily transmitted throughthe fuel cell stack. Alternately, the support function could be providedwith grooved blocks of a non-conductive material or similar featureswhich could be formed in the seals to replace the in-situ configurationprovided by the transverse support flange.

When using this configuration, these adjacent regions must be insulatedsince they are at different electrical potentials. Various suitablemeans are available such as the use of a non-conductive coating such asthat disclosed in U.S. application Ser. No. 10/132,058 entitled “FuelCell Having Insulated Coolant Manifold” filed on Apr. 25, 2002 which iscommonly owned by the assignee of the present invention and thedisclosure of which is expressly incorporated by reference. Alternately,a film of non-conductive plastic tape my be interposed for providingelectrical isolation therebetween.

The present invention provides a two piece bipolar plate assembly havinga coolant flow field formed therebetween. The configuration of thevarious flow fields are such that the bipolar plate assembly may be aformed of relatively thin material, and still support the requiredcompressive loads of the fuel cell stack. Furthermore, the presentinvention provides much greater design flexibility in terms of flowfield options. In this regard, the present invention provide animprovement in the gravimetric and volumetric power densities of a givenfuel cell stack as well as significant material and cost savings.

The description of the invention set forth above is merely exemplary innature and, thus, variations that do not depart from the jest of theinvention are intended to be within the scope of the invention. Suchvariations are not to be regarded as a departure from the spirit andscope of the invention.

1. A separator plate for a bipolar plate assembly for a fuel cell comprising: a thin plate having an inlet margin including a pair of reactant gas inlet headers and a coolant inlet header formed therethrough, an exhaust margin including a pair of reactant gas exhaust headers and a coolant exhaust header formed therethrough and a flow field formed on a major face of said thin plate, said flow field including a first set of flow channels, each of said first set of flow channels having an inlet leg with a first longitudinal portion in fluid communication with one of said pair of reactant gas inlet headers and a first transverse portion adjacent said coolant inlet header, a serpentine leg having a first end in fluid communication with said first transverse portion and a second end, and an exhaust leg having a second transverse portion adjacent said coolant exhaust header and in fluid communication with said second end of said serpentine leg and a second longitudinal portion in fluid communication with one of said pair of reactant gas exhaust header, wherein at least one of said first transverse portion of said inlet leg adjacent said coolant inlet header and said second transverse portion of said exhaust leg adjacent said coolant exhaust header defines an undulating flow channel.
 2. The separator plate of claim 1 wherein said undulating flow channel in said first set of flow channels is formed in said first transverse portion of said inlet leg.
 3. The separator plate of claim 1 wherein said undulating flow channel in said first set of flow channels is formed in said second transverse portion of said exhaust leg.
 4. The separator plate of claim 1 wherein said undulating flow channel comprises at least two undulations.
 5. The separator plate of claim 1 wherein said pair of reactant gas inlet headers are located laterally on either side of said coolant inlet header and said pair of reactant gas exhaust headers are located laterally on either side of said coolant exhaust header.
 6. The separator plate of claim 1 wherein said flow field further comprises a second set of flow channels, each of said second set of flow channels having an inlet leg with a first longitudinal portion in fluid communication with said one of said pair of reactant gas inlet headers and a first transverse portion adjacent said coolant inlet header, a serpentine leg having a first end in fluid communication with said first transverse portion and a second end and an exhaust leg having a second transverse adjacent said coolant inlet header portion in fluid communication with said second end of said serpentine leg and a second longitudinal portion in fluid communication with said one of said pair of reactant gas exhaust headers, wherein at least one of said first transverse portion of said inlet leg and said second transverse portion of said exhaust leg defines an undulating flow channel.
 7. The separator plate of claim 6 wherein said undulating flow channel in said second set of flow channels is formed in said first transverse portion of said inlet leg.
 8. The separator plate of claim 6 wherein said undulating flow channel in said second set of flow channels is formed in said second transverse portion of said exhaust leg.
 9. The separator plate of claim 6 wherein said undulating flow channel comprises at least two undulations.
 10. The separator plate of claim 6 wherein said pair of reactant gas inlet headers are located laterally on either side of said coolant inlet header and said pair of reactant gas exhaust headers are located laterally on either side of said coolant exhaust header.
 11. The separator plate of claim 1 wherein each of said first set of flow channels further comprise a third transverse portion adjacent said coolant inlet header having a first end in fluid communication with said first longitudinal portion, a second serpentine leg having a first end in fluid communication with said third transverse portion and a second end and a fourth transverse portion adjacent said coolant exhaust header and in fluid communication with said second end of said second serpentine leg and said second longitudinal portion, wherein at least one of said third transverse portion of said inlet leg adjacent said coolant inlet header and said fourth transverse portion of said exhaust leg adjacent said coolant exhaust header defines an undulating flow channel. 